For a number of years, oxygen sensors have been employed in motor vehicles in conjunction with the utilization of three-way catalyst systems for the treatment of exhaust gases from internal combustion engines. Generally, the oxygen sensor and three-way catalyst are coupled with a feedback control system of an air/fuel ratio metering device to control the stoichiometric air/fuel ratio being fed to the internal combustion engine at any particular time. If the oxygen sensor detects too much oxygen in the exhaust gases after passage of those gases through the three-way catalyst, more fuel is supplied to the engine. On the other hand, if the oxygen sensor detects an insufficient amount of oxygen in the exhaust gases, then the feedback control system decreases the amount of fuel being fed to the internal combustion engine.
One type of oxygen sensor which has been developed for such an application uses zirconium oxide, zirconia, as the sensing element. This material has been used in oxygen sensors for millions of automobiles now on the road in the United States.
As an alternate to the zirconium oxide sensor, a titanium dioxide, titania, type of oxygen sensor has been developed by Ford Motor Company. The particular titanium dioxide sensor developed by Ford Motor Company is covered by a number of U.S. patents, see, for example, U.S. Pat. Nos. 3,893,230; 3,911,386; 3,932,246; 3,933,028; 3,959,765; 4,001,758; and 4,151,503.
These patents are representative of the patents Ford Motor Company has obtained in the field of titanium dioxide oxygen sensors. However, these titanium dioxide sensors have a slower response time than the zirconia sensor developed by others. By slower response time we mean that it takes a longer period of time for the titanium dioxide oxygen sensor to detect a switch from oxygen-rich to oxygen-lean conditions, or vice-versa, than does the zirconia sensor.
This slower response of the titania sensor does not have any effect on the feedback control system and, thus engine operation, as long as the response of the feedback control system is limited by a slow fuel metering device such as a feedback carburetor. The inherent mechanical limitations of the carburetor make this device much slower responding than the titania sensor so that the system itself does not in any manner note the slower response time of the titania sensor when this sensor replaces a zirconia type sensor. However, when the feedback control system uses a fast reponse fuel metering device, such as an electronic fuel injection system, then the feedback system could be hampered by the slower response time of a titania oxygen sensor.
The present development was brought about in order to obtain a titania sensor with a faster response time than the previously known titania sensors. It is a principal purpose of our invention to teach titanium dioxide sensors which have response times sufficiently fast that the fuel injected type of feedback system would not notice a delay in receiving a control signal because of a delay in the oxygen sensor's ability to detect changes from oxygen-rich to oxygen-lean conditions and vice-versa. We accomplished this purpose by developing titanium dioxide sensors of the so-called thick film variety which will hereafter be described in greater detail.
The ceramic titanium dioxide (TiO.sub.2) materials developed by Ford for oxygen sensing applications, as set forth in the previously cited Ford-assigned patents, have a density in the range from 65-80% of the theoretical value. The microstructure of this type of material consists of interconnected TiO.sub.2 particles with a size of a few micrometers and separated by interconnected pores with a similar size. The addition of catalytic metals into the porous TiO.sub.2 ceramic accelerates the oxygen transfer process between the solid and the ambient gas and leads to a substantial improvement in the sensor response time in detecting changes in the oxygen content of exhaust gases.
Our studies have indicated that the response time of noble metal impregnated ceramic TiO.sub.2 sensors is limited to a large degree by the gas transport process through the pores of the ceramic sensor material. One approach for decreasing the effect of this gas transport process is to decrease the thickness of the TiO.sub.2 material; for example, by replacing the ceramic material with a film. If the film is made dense, its thickness must be kept small, less than 10 um (um means micrometers), otherwise oxygen diffusion in the bulk of the solid would result in unacceptably long response times. On the other hand, films with a small thickness, less than 10 um, may not have the required durability since, for example, some erosion of the film would be caused by the constant movement of exhaust gases of an engine thereover. It is thus desirable to prepare films thicker than 10 um, preferably thicker than 20-30 um. However, the response time of sensors using such thick films would be unacceptably long (due to bulk diffusion of oxygen) unless the film has an optimized porosity and microstructure that minimizes the contribution of bulk diffusion and gas transport through the pores to the sensor response delay.
The preparation of films of metal oxides, and TiO.sub.2 in particular, has been discussed extensively in the literature. Reported preparation methods include thick film techniques using appropriate printable inks, sputtering, thermal evaporation, and chemical vapor deposition. However, most of the prior art TiO.sub.2 film preparation methods are not suitable for oxygen sensor applications because they do not provide the film composition, microstructure, and electrical and mechanical properties required for fast responding oxygen sensors. For example, sputtering deposition tends to give dense films which, as discussed above, would not provide fast oxygen sensors unless their thickness is kept undesirably small. Amorphous films with densities much smaller than theoretical (in the range of 50-80%) have been prepared by various techniques; however, these films are unstable at the high temperatures of the automotive exhaust and do not generally have the required electrical properties. High temperature annealing to convert them to the stable rutile structure has the tendency to change them to dense materials.
A common and commercially used technique for thick film preparation is screen printing from inks. U.S. Pat. No. 4,335,369 to Taniguchi et al describes a method of preparing thick TiO.sub.2 films by screen printing from TiO.sub.2 pastes for oxygen sensing applications. Sensors using these films were found to be faster than those employing ceramic TiO.sub.2 materials. The method described in the above patent, however, is tedious and time consuming because it was found that the optimum film thickness of 50-100 um with the required microstructure could not be obtained unless the TiO.sub.2 paste was deposited in approximately 15 um steps, each step followed by a long drying period (more than one hour).
Another commercially important film preparation technique is the chemical vapor deposition (CVD). Metal oxides including TiO.sub.2 have been prepared by CVD from several inorganic and organic compounds. Of the organic compounds, tetraisopropyl titarate Ti(C.sub.3 H.sub.7 O).sub.4 has been used for the preparation of dense, hard and abrasion-resistant titanium oxide (with unspecified Ti/O ratio) protective coatings of glassware. These materials, however, are amorphous, unstable at elevated temperatures, and do not have the electrical properties required for oxygen sensor applications.
S. Sakurai and M. Watenabe (Rev. Elect. Comm. Lab. 11, 178, 1963) decomposed Ti(C.sub.3 H.sub.7 O).sub.4 [and Ti(C.sub.2 H.sub.5 O).sub.4 ] in vacuum above 900.degree. C. to obtain TiO.sub.2 films with the rutile structure. These films, however, were found to be very dense (99% of the theoretical density); this method of TiO.sub.2 film preparation is not therefore suitable for obtaining fast TiO.sub.2 oxygen sensors.
More recently, Yokozawa et al (Japan J. Appl. Phys. 7, 96, 1968) investigated the preparation of thin titanium oxide films on silicon wafers from Ti(C.sub.3 H.sub.7 O).sub.4 for possible use in photo-etching technology. They found that thermal decomposition of Ti(C.sub.3 H.sub.7 O).sub.4 in an atmosphere of nitrogen and oxygen in the range of 320.degree.-540.degree. C. gave films consisting of very fine crystallites of anatase phase. These films are apparently unstable since the anatase phase films were found to possess the remarkable property of being easily etched (4-60 A.degree./sec.) by a diluted HF solution; in constrast, normal TiO.sub.2 cannot be etched with any known etching agent (except KOH above 150.degree. C.).
It is thus desirable to develop TiO.sub.2 thick film preparation techniques that provide the microstructure and electrical properties required for fast responding oxygen sensors. The technique developed should be simple and not time consuming so as to be acceptable for high volume production of oxygen sensors. These are some of the objectives that this invention attempts to fulfill.